Liu, TengTengLiuTOZZI, PaoloPaoloTOZZIWang, Jun-XianJun-XianWangBrandt, William N.William N.BrandtVIGNALI, CRISTIANCRISTIANVIGNALIXue, YongquanYongquanXueSchneider, Donald P.Donald P.SchneiderCOMASTRI, AndreaAndreaCOMASTRIYang, GuangGuangYangBauer, Franz E.Franz E.BauerPaolillo, MaurizioMaurizioPaolilloLuo, BinBinLuoGILLI, RobertoRobertoGILLIWang, Q. DanielQ. DanielWangGiavalisco, MauroMauroGiavaliscoJi, ZhiyuanZhiyuanJiAlexander, David M.David M.AlexanderMAINIERI, VINCENZOVINCENZOMAINIERIShemmer, OhadOhadShemmerKoekemoer, AntonAntonKoekemoerRISALITI, GuidoGuidoRISALITI2020-08-042020-08-0420170067-0049http://hdl.handle.net/20.500.12386/26693We present a detailed spectral analysis of the brightest active galactic nuclei (AGNs) identified in the 7Ms Chandra Deep Field-South (CDF-S) survey over a time span of 16 years. Using a model of an intrinsically absorbed power-law plus reflection, with possible soft excess and narrow Fe Kα line, we perform a systematic X-ray spectral analysis, both on the total 7Ms exposure and in four different periods with lengths of 2-21 months. With this approach, we not only present the power-law slopes, column densities {N}<SUB>{{H</SUB>}}, observed fluxes, and absorption-corrected 2-10 keV luminosities L <SUB> X </SUB> for our sample of AGNs, but also identify significant spectral variabilities among them on timescales of years. We find that the {N}<SUB>{{H</SUB>}} variabilities can be ascribed to two different types of mechanisms, either flux-driven or flux-independent. We also find that the correlation between the narrow Fe line EW and {N}<SUB>{{H</SUB>}} can be well explained by the continuum suppression with increasing {N}<SUB>{{H</SUB>}}. Accounting for the sample incompleteness and bias, we measure the intrinsic distribution of {N}<SUB>{{H</SUB>}} for the CDF-S AGN population and present reselected subsamples that are complete with respect to {N}<SUB>{{H</SUB>}}. The {N}<SUB>{{H</SUB>}}-complete subsamples enable us to decouple the dependences of {N}<SUB>{{H</SUB>}} on L <SUB> X </SUB> and on redshift. Combining our data with those from C-COSMOS, we confirm the anticorrelation between the average {N}<SUB>{{H</SUB>}} and L <SUB> X </SUB> of AGN, and find a significant increase of the AGN-obscured fraction with redshift at any luminosity. The obscured fraction can be described as {f}<SUB>obscured</SUB>}≈ 0.42 {(1+z)} 0.60STAMPAenArticle10.3847/1538-4365/aa78472-s2.0-85030266777000411595000003https://arxiv.org/abs/1703.00657https://iopscience.iop.org/article/10.3847/1538-4365/aa78472017ApJS..232....8LFIS/05 - ASTRONOMIA E ASTROFISICAPE9 Universe sciences: astro-physics/chemistry/biology; solar systems; stellar, galactic and extragalactic astronomy, planetary systems, cosmology, space science, instrumentation